Isotactic polypropylene is a well known article of commerce prepared by the polymerization of propylene in the presence of a catalyst comprised of an alkyl aluminum chloride and titanium tetrachloride. Such polypropylene is well suited to the preparation of packaging films and a large industry has grown up about polypropylene packaging films.
Recent work by certain polymer manufacturers has led to the development of a process for polymerization of propylene in the presence of metallocene compound catalysts. The resulting polymers, while quite similar to conventional isotactic polypropylene in many respects, do exhibit some unique properties and improvements in other properties as compared to the conventional polypropylenes.
Metallocene catalysts are defined as bridged bisdicyclopentadienyl or bisindenyl Group 4,5 or 6 transition metal dihalide derivatives. Specific metallocene catalysts known to be useful for producing polypropylene are discussed, inter alia, in EPA 485,820; 485,821; 485,822; 485,823; 518,092 and 519,237 and in U.S. Pat. Nos. 5,145,819 and 5,296,434. Other references that discuss the metallocene catalyzed process include EPA 351,932 and U.S. Pat. Nos. 5,055,438; 5,234,800; 5,272,016; 5,272,236 and 5,278,272. All of the cited documents are incorporated hereinto by reference.
The polymers resulting from metallocene catalysis are said to be of extremely uniform steric structure. Depending on the specific metallocene employed, the polymer can be of an isotactic structure wherein the pendant methyl groups on the polymer chain are located on alternating carbon atoms and are alternately oriented above and below the plane of the chain. With another metallocene, the polymer can be of syndiotactic structure wherein all of the pendant methyl groups are still located on alternate carbon atoms, but they are disposed predominantly on the same side of the chain.
In addition to the high degree of stereoregularity of the polymer chains, there is a very high degree of uniformity of molecular weight among the polymer chains, i.e., the molecular weight distribution is narrower than that found in conventional isotactic polypropylene. Thus, whereas a conventional polypropylene has a polydispersity (i.e. the ratio of weight average molecular weight to number average molecular weight) on the order of about 7, a metallocene catalyzed polypropylene prepared by the same manufacturer has a polydispersity on the order of about 3. Likewise, in the manufacture of copolymers or terpolymers, the metallocene catalyst leads to a more even distribution of the comonomer or comonomers throughout the product. Yet another good feature of the metallocene polymers is that they have a lower concentration of low molecular weight, xylene soluble materials.
The narrower molecular weight distribution and the more even distribution of the comonomer leads to greater uniformity of the properties of the product. Thus, the metallocene catalyzed products exhibit a sharper melting point and a more uniform intrinsic viscosity throughout the product. These property improvements translate into improved flow properties and thus to improved processing during film extrusion and drawing. Processing is thus significantly facilitated.
However, in attempting to manufacture oriented packaging films with the metallocene catalyzed polypropylene in conventional processes and equipment, it has been found that the cast films exhibit very poor properties in terms of their optical qualities. Specifically, the films cast preparatory to drawing to effect orientation are extremely hazy, bordering on being translucent. In addition, they have a rough, sand paper-like surface.
It is the object of this invention to provide a polymer composition comprised of metallocene catalyzed polypropylene that can be employed in the manufacture of polypropylene film that does not exhibit the shortcomings of metallocene catalyzed polypropylene mentioned above.
It is a further object of the invention to provide superior polypropylene films based on metallocene catalyzed polypropylene and having a combination of improved properties as compared with polypropylene films heretofore known in the art.